As electronics technologies become increasingly integrated, a wider variety of portable electronic devices is enabled. A critical associated technology allowing portability is energy storage, particularly, battery technology. Without an energy source, portable devices are useless. The main concerns with regard to portable devices for any battery or battery pack are physical size, weight, energy capacity, and predictable behavior. In general, it would be desirable to have a small, lightweight, high capacity battery, and while small size and light weight generally go together, the two are at odds with battery capacity. In general, the larger the battery, the higher the capacity. The designers of a portable system must make a compromise between size and capacity, although newer chemistries are allowing for smaller batteries with higher capacities. In fact there is currently a great deal of work being done to improve battery capacity. However, predicting the life of the battery while it is powering a portable device is difficult, at best.
Since batteries have a finite energy capacity, the user of a portable device runs the risk of having the battery suddenly running out of power in the absence of some means to recognize a low capacity condition. Measuring the remaining battery capacity in a battery during use is not as straight forward as measuring the fuel level in an automobile. Battery parameters are fraught with non-linearities, as is well documented in the art. For example, the voltage of a battery is affected by the amount of current being provided (or accepted), the temperature of the battery, the age of the battery, the specific chemical recipe used, the state of charge of the battery, and the ratio of the battery current to the battery capacity, to name a few.
Accordingly, early attempts to warn a user of a low capacity condition based on only one or two parameters were unreliable. Methods such as observing the battery voltage until it dropped to a preselected threshold would work well under the right conditions, but were grossly inaccurate under less favorable conditions. This typically was not seen as a great disadvantage at first, since the complexity of the portable devices was, by comparison, very low. However, with the proliferation of computers into the portable realm, it has become increasingly necessary to have a reliably precise method to measure battery capacity. For example, a portable computer user must be warned when the battery is going to run out so that any work in progress may be saved. In addition to a warning, users would like to know the relative capacity of a battery. For example, a user may have several batteries for a given device. If the batteries are in unknown states of charge, i.e. they were not just recharged, it is impossible to tell which battery will give the longest period of operation.
In response, the market has demanded more precision from the manufacturers in determining remaining battery capacity, as well as a state of charge indicator. Many manufacturers have responded to the market with what is commonly referred to as a smart battery. One of the first attempts to make an ultra-precise measurement of the remaining capacity of a battery involved measuring the current through the battery and integrating the current flow with respect to time. This method uses a counter keeps track of the integrated value, and provides the portable device with a count upon request from the device. This method requires a stable, accurate time keeping method, as well as a very precise means for measuring current. It is the basis for virtually all current smart battery circuits. Some improvements have been made, such as including a microprocessor programmed to adjust the integrated value based on temperature and other conditions. In some instances the software developed for these more advanced battery packs is complex. Accordingly, the smart battery has met with some success in the marketplace. However, the precision provided by this method comes at a significant cost, occasioned by the need for additional circuitry.
Since batteries are an accessory manufactured for a portable device, they are expected to come at as low a cost as possible. Customers may be willing to spend a little more money for a given device based on some unique features, but rarely for the associated battery. In response, many device manufacturers offer both smart and regular batteries for their products, and leave the decision up to the consumer. However, manufacturers have failed to recognize that the ultra-precision of the current smart batteries is more than required by the typical consumer. For example, consumers know to refill their automobile fuel tanks when the indicator drops to a low level. An indicator that would tell them there were 1.234 gallons left is not necessary. While battery parameters may be non-linear, a reasonable estimation of capacity is still possible. Therefore there exists a need for a reliable state of charge indicator that is also cost effective.